1. Field of the Invention
The present invention relates generally to a structure and method for fabrication of vapor cells adapted for use in making chip-scale atomic clocks (CSACs) via wafer-scale micro-machining processes.
2. Discussion of the Prior Art
The need for more and more precise and stable time-keeping for a wide variety of applications has been on the rise, particularly in applications such as digital communications, global positioning systems (GPS) and, more critically, for security and identification applications such as friend-or-foe (IFF) communications.
There are a wide variety of potential applications for enhanced time or frequency reference signal sources, which may be referred to as time bases or clocks. One example of the need for precise, stable time keeping is found in the development of enhanced, jam-resistant GPS receivers. The signals broadcast by GPS satellites are extremely low in power, making the GPS receivers highly susceptible to intentional jamming signals as well as to unintentional interference from sources transmitting in the same frequency band. For example, some GPS signals are transmitted over a wide bandwidth, making them considerably less susceptible to jamming than normal GPS signals. Typically, however, these broadband signals incorporate a code that repeats only every seven days, so that broadband receivers usually have to first lock onto the normal signal, and this eliminates the anti-jam advantage of the larger-bandwidth signal. If the broadband receiver's local clock were capable of determining the time to within 1 millisecond (ms) over several days, its search for the GPS signals would be narrowed so the receiver could, theoretically, lock onto the broadband signal directly, without first having to acquire the normal signal. Thus, if a more accurate clock were available, the receiver would be significantly more resistant to jamming.
Three important characteristics are necessary to realize a ‘good’ time base or clock: (1) long and short-term frequency stability (usually measured in Allan variance and phase noise of the frequency source); (2) physical size of the clock; and (3) the power consumed by the clock. Historically (and mainly to satisfy criterion 1), clocks based on electromagnetic oscillations of atoms have provided the most precise method of timing events lasting longer than a few minutes. So precise are these “atomic” clocks, that in 1967 the second was redefined to be the duration required for a cesium (Cs) atom in a particular quantum state to undergo exactly 9,192,631,770 oscillations. While the long-term precision of atomic clocks is unsurpassed, the size and power required to run them has prevented their use in a variety of areas, particularly in those applications requiring portability or battery operation. The NIST F-1 primary standard, for example, occupies an entire table and consumes several hundred watts when operating. The state of the art in compact commercial atomic frequency references is rubidium (Rb) vapor-cell devices with volumes near 100 cm3 operating on a few watts of power; such references cost about $1,000.00 USD.
Recently, miniature atomic clocks have been based on Microelectromechanical systems (MEMS) technology which offers advantages such as smaller size, an improvement in the device power usage due to reduced parasitic heat dissipation (as the heat lost to the environment via the device surface is smaller), and high-volume, wafer-based production methods, which may substantially reduce cost. In spite of these advantages, the power consumed by currently envisioned MEMS-based atomic clocks hasn't been reduced enough to permit their use in applications such as portable battery operated systems in long-term operations, including, for example, week-long missions for the military, months-long working of communication base units or even year/decade long operation for sensor node applications.
Prior art atomic clocks typically include a physics package, which is the heart of the clock and contains an atomic (usually Rb or Cs) vapor cell that acts as a frequency reference to determine the clock output frequency.
Solid state resonators (such as RF resonators based on quartz and silicon) are portable and energy efficient and so are often used in wrist watches and the like, but cannot provide an adequate reference signal because they have observable and random aging effects which cause their frequencies to shift in a non-predictable manner.
Stable frequency sources are extremely important for communication systems for civil and military applications, and for sensor stability for long-term operation of sensor nodes. Considerable work has been done in the last few years to realize miniaturized atomic clock systems or chip-scale atomic clocks (CSACs) demonstrating potential for portability and low power operation. Low operation power and size of the CSACs are required for portability, whereas good short and long-term stability and low cost of fabrication are essential to ensure applicability in a wide variety of markets.
The frequency stability of atomic clocks is based on transitions between the well-defined ground state hyperfine levels of alkali atoms such as rubidium (Rb) or cesium (Cs). The physics package of an atomic clock consists of alkali metal atoms enclosed in a vapor cell so that the atomic resonance is excited and interrogated by an RF local oscillator about a frequency that corresponds to the hyperfine energy difference in the ground state of atoms.
The vapor cells of CSACs use sealed micromachined cavities to enclose the highly reactive alkali metals (such as rubidium—Rb and cesium—Cs) in a buffer gas composition. In addition, since the size of the vapor cells are very small (of the order of mm3), the buffer gas pressure and composition have to be optimized to serve the two important purposes of creating an inert ambient environment for the alkali metals, and maximizing the coherence lifetimes of the atoms by decreasing their effective wall relaxation., in turn reducing the linewidth of the hyperfine absorption.
The use of highly reactive and low melting alkali metals and filling the vapor cells with the optimum pressure and composition of buffer gases thus impose a MEMS packaging challenge. The fabrication of MEMS vapor cells so far has involved anodic bonding between micromachined silicon cavities and Pyrex glass. Since anodic bonding requires high-temperature (˜400° C.) processing, whereas the melting points of alkali metals are much lower (Rb ˜39.3° C., Cs ˜28.4 ° C. at 1 atm), the alkali-metal and buffer gases cannot be placed inside the cavities before the bonding process reliably. Knappe, et al, have tried to solve this problem by in-situ fabrication of the alkali metals from high temperature reaction of metal hydrides, metal chlorides and/or metal hydroxides during bonding. This can lead to residual impurities that cause long term drifts of the hyperfine resonance frequency. Lee, et al, have demonstrated a method of interconnecting the cavities using micromachined channels, and parallel filling after vapor cell formation. However isolation of the cells from each other and dicing requires the use of a wax-sealing, which leads to low yield, and requires bulk rubidium delivery, which is inefficient and results in uncontrolled delivery of rubidium in each vapor cell.
An alternative to using buffer gas to increase the coherence life time is to use a thin and uniform (or monolayers) of wall coating of materials such as Teflon (used in hydrogen maser frequency references), long chain alkanes (such as n-tetracontane—a component of paraffin wax), or some alkynated silanes (used in alkali metal frequency standards), as reported by Frueholtzs, et al and Sagiv, et al. However, the stringent requirements for the quality and apparatus needed for formation of wall coating is currently not compatible with MEMS processing. Furthermore, alkylated silanes have been shown to degrade over long-term operations directly affecting the long-term stability of the atomic clock system.
There is a need, therefore, for a structure and method for reliable fabrication of vapor cells adapted for economical use in making chip-scale atomic clocks (CSACs) via wafer-scale micro-machining processes that overcomes the problems with the prior art.